Polarization plane rotator for microwave energy



G. T. RADO April 29, 1958 POLARIZATION PLANE ROTATOR FOR MICROWAVE ENERGY Filed Aug. 18, 1952 l. U E T N N vR E O U v D N A Y I R A L w T. 4 U S E C G M D R O O T E G m decay to l/e' of its nited States (Granted under Title 35, S. Code (1952), see. 266)" This invention relates to improvements in a polarizat1on planerotator for electromagnetic waves. The device of the present invention utilizes the Faraday rotation principle and may be used as a high'speed microwave switch for'use fasa transmitter-receiver switch in microwave frequency communication and radar circuits, in one-way transmission systems, microwave circulators and electrically controlled variable attenuators and modulators.

Prior art devices embodying the Faraday rotation prin- 'ciple for polarization plane rotation comprise metallic wave guides of circular cross-section containing cylindrically shaped ferromagnetic slugs within the wage guide and solenoidal windings encircling the waveguide walls in the vicinity of the slug; Provisions maybe made to reduce reflections at the flat surfaces of the slug. When a direct current is passed through the windings encircling the wave guide walls, the ferroma'gnetic slug is magnetized in a direction substantially parallel to the axis of the guide. This causes a plane polarized wave propagated Vthrough-the guide to split'into positive and mega;

tive circularly polarized components having different refractive indic es. This action causes the two circular components to travel with different velocities in the ferromagnetic medium and upon emerging 'from'the medium, they are equivalent to a plane polarized wave whose plane of' polarization has been rotated with respect to the incident polarization. Since the difference in velocities between the rotations-produced are'large'ev'en when'the effective magnetic field is small. Ifthe frequency is sufficiently high in comparison to certain characteristic values determined by'the saturation magnetization and the applied 4 field, then the energy loss is often quite small.- The rotation obtained has been found to be directly proportional to the magnetization of the ferromagnetic materialup to the point of saturation and approximately proportional to the leng'thof ferromagnetic cylinder used. In addition.

the components is appreciable in the microwaveregion,

2,832,938 Patented Apr. 29, 1958 2 solenoid surrounding the conventional circular-metallic Wave guide cannot penetrate rapidly enough through the metallic wave guide into the ferromagnetic croe contained within the guide. j "f The difliculties encountered with prior art devices of the above type were first determined by applicant to be due to eddy current shielding by the metallic wave guide walls. In other words, the transient skin effect prevents rapid penetration of the field through the metallic guide.

It is, therefore, an object of this invention to provide an improved polarization plane rotator for use with microwave energy that utilizes the Faraday rotation of the plane of polarization of microwaves. i 1

It is another object of the invention to provide a microwave switch that has a "switching-time-constant of the order of a microsecond or less.

Other objects and features of the present invention will become apparent upon consideration of the following detailed description taken in connection with the accompanying drawings which illustrate certain embodiments of the invention. It is to be expressly understood, however, that the drawings are designed for purposes of illustration only and not .as a definition of the limits of the invention, reference for the latter purpose is to be had to the appended claims.

Figure 1 is a partial cross-section and partial perspective view of a portion of one embodiment of the present invention,

Figure 2 is a view similar to that of Figure 1 showing a different embodiment of the present invention,

Figure 3 shows an embodiment of the present invention utilized in practice as a microwave switch.

According to the principles of this invention, a polarization plane rotator of microwave energy is provided by utilizing a circular wave guide section in which a ferromagnetic slug, axially aligned with the section, is embedded concentrically within a dielectric filling medium.

This section is encased within a solenoid. When a D. C. voltage pulse is applied acrossthe solenoid windings, an axial magnetic field is produced. The plane of polariza- .tion of plane polarized microwaves within the section is caused to rotate for reasons stated above in the general theoretical discussion. This rotation can be controlled by varying the voltage applied to the solenoid, the magnetic characteristics and length of the ferromagnetic slug used and the dielectric constant of the ferromagnetic jmaterial. Inorder to decrease the delay time due to transient skin effects in the wall of the wave guide ,which effects prevent a rapid penetration of the magnetic field the rotation depends on the dielectric constant of the.

ferromagnetic medium. a V

Present microwave switches utilizing the Faraday rotationof the plane ofpolarization of microwaves are deficient in their operation when high speed switching is desired.j:This deficiency is 'due to'the switchingetimeconstant.of about 150 microseconds in present devices. Such a time interval is far too long for effective utilization of such a switch in radar-and some microwave communication systems. We define the1"switching-time-constant as the time required for the R. F. power transmitted to on value when the switch is used as "a turn-off-device. An analogous definition is applied when thQ'SWitCh is used-as a-turn-on device.

Previous attempts to-utilze the Faraday rotation-effect in therealization of a high-speed microwave switch were abandoned'due to the belief that the relatively long time delay in the switch operation was: inherent in the ferro- .-magnetic core used. The Applicant suggested that this delaywas primarily due to the fact: that the magnetic 1 field pulse generated by a currentpulse applied to.a

through the wave guide wall, it is proposed to provide a low eddy current loss support means for supporting the ferromagnetic slug within the solenoid and minimizing the eddy current loss in the guide member. Two solutions are proposed. The first embodies cutting a thin slot substantially parallel to the axis of the wave guide and at least equal in length .to the ferromagnetic core within the wave guide thereby providing an open eddy current circuit. The other is to use the ferromagnetic core itself as the wave guide and space the core from the solenoid by means of a dielectric material. 1

With special reference to the drawings, Figure 1 shows a first embodiment of the present invention. The rotator represented in general by reference number 10 comprises a cylindrical, metallic wave guide wall 12, a low loss dielectric filling material 14, such as polystyrene or Teflon, -within said wave guide, and a ferromagnetic core 16.

This core 16 is of a ferromagnetic materialof low conductivity to prevent eddy current losses. A typical family of substances used are the ferrites, which have the general formula M0Fe O where M stands for a divalent metal, such as, for example, nickel, manganese and magnesium.

A thin, longitudinal slot 18, having a width on the order of .01 inch and a minimum length'equal to the axial length of the ferromagnetic core, is cut out of the wave guide wall along an axis substantiallyparallel .to the wave guide axis; .A solenoid 20 is woundfarouud the periphery ofthe rotator. A representative prior artswitch operating in the 3 cm. wavelength region' whichiis adaptedto be modified by the teachings of the present invention utilizes a solenoid of aboutSOO turns, having alength of 1 inches and an inductanceof the order .ofl milliheni'y and a magnetic core having a dielectric constant of about 10.

magnesiumferrite, and the current through the solenoid is about 1 or 2 amperes or-larger. Such a device causes 6 a 90 degree rotation of the plane of polarization of microwave energy propagated through the switch.

Figure 2 shows another embodiment ofthe present invention. Rotator 30 comprises a dielectric material 34 encircling a ferromagnetic core 36. A solenoid 40 is "a ser s,

. and by or for the Government of the United States Its dimensions include acore length of 1.26 inches 7 and diameter of .23 inch, and an inside diameter :of the wave guide of .681 inch. The core material is essentially magnetizing coil concentric with-ferromagnetic cylindrical core 36 and having a radius large compared to that of the ferromagnetic core 36. These characteristics assure a short time-constant coil producing a fairly uniform axial magnetic field, and prevent attenuation of the R. .F. field by themetallic coil because .the R. F. field decays exponentially outside the ferromagnetic cylinder if thelatter acts as a wave guide.

The embodiment depicted in Figure 1 represents one extreme-solution of the problem solved in this invention and the embodimentof Figure2 represents the opposite extreme solution. It is understood that the elimination of anyportion of the metallic wave guide walls between the two extremes shown also provides a solution to the problem of cutting down the delay time inherent in .the

operation of devices of the class claimed, provided the portion cut extends from'a point transversely aligned with one end of-thecore at least to a point transversely aligned with the other end of the coreso that no closed circumthe polarized energy propagated from left "toright through guide50, passes through joints 52 and '54 as well-asrotator 10 or 30'but since guide "56.is oriented at 90, degrees 'to guide 50, extremely little energy passes intoguider56. Upon the initiation of aD. Cgvoltagepulse through solenoid 2 0 or 40, the E=vector of the energy propagated through guide 50 isrotated 90 degrees in circular'guide 10 or 30, thereby enabling the energy to pass through wave guide 56 relatively unimpeded. The provision pr either axially longitudinal slot 18 or a suitably'tl'iick dielectric medium and no metallic waveguide wall enables the switchvto respond very rapidly It is-understood that rotators .10 or 30 could also be used ,as an 'on-off switch rather .than as an foif-on switch by orienting rectangular guides 50and parallelto each other. j j

The invention described herein may be manufactured of America for governmental purposes without the 'payment of any royalties thereon or therefor.

What is claimed is:

' Y 1. A high speed device for rotating the plane of polarization of propagated electromagnetic wave energy in a conductive wall waveguide-system comprising a wavcguide :section capable of supporting two orthogonal dominant modes of a propagated wave and adapted for incorporation in said waveguide system; a ferromagnetic core element; said'core element'being active in the Faraday rotational sense'and centrically disposed within saidwaveguide section; magnetic field producing means for producing a unidirectional axial field within said waveguide section; said field producing means being disposed .outside said waveguide section "such that lines of force of the axial field produced thereby intersect said core element; at least a portion of the wall of said waveguide section, which isidisposed in the vicinity of said core'element, being adapted to provide a substantially greater impedance to the flow of circumferential eddy currents with respect to the impedance presented to current flow in a circumelement being active in the Faraday rotational sense and centrically disposed within said waveguide section; magnetic field producing means for producing a unidirectional axialfield within said waveguide section; said field producing .means being idsposed outside said waveguide section such that lines of'force of the axial field produced thereby intersect said core element; said waveguide section havingatleast a portion of the wall thereof, cutaway; said cutaway portion being coextensive in length with said core element .such that said waveguide section provides a 'substantiallygreater impedance to the flow of circumferential eddy currents with respect to the impedance presented to current flow in a circumferential direction by the conductive wall of said waveguide system.

3. A high speed device for rotating .theplane ofpolari- ;zation :of propagated electromagnetic wave .energyin a conductive wall waveguide system comprising a waveguide-section capable of supporting two orthogonal dominantmodes of a'propagated wave and adapted for incorporation in said waveguide system; an elongated ferromagnetic coreelement of predetermined length; said core element being active in the Faraday rotational senseand centrically disposed within said waveguide section; magnetic-field producing means for producing a unidirectional axial field within said waveguide section; said field producing means being disposed outside said waveguide section such that lines of force of the axial field produced :provides a substantially greater impedanceto thefiow of circumferential eddy currents with respect to the impedance presented to currentflow in a circumferential direction'by the conductive wall of said waveguide system.

4. A high speed device for rotating the plane of polarization of propagated electromagnetic wave energy'in a-conductive wall waveguide .system comprising 'a waveguide section capable of supporting two orthogonal dominant modes of a propagated wave and adapted forincorporation in said waveguide system; a core element active in the Faraday rotational sense; said core element being lcentrically disposed within said waveguide section; magnetic field producing means for producing a unidirectional axial field within said *waveguide section; said field producing means being disposed outside said waveguide section such that lines of force of the axial field produced thereby intersect said core element; at least a portion of the wall of said waveguide section, which is disposed in the vicinity of said core element, being adapted to provide a substantially greater impedance to the flow of circumferential eddy currents with respect to the impedance pre' sented to current flow in a circumferential direction by the conductive wall of said waveguide system.

References Cited in the file of this patent UNITED STATES PATENTS 2,197,123 King Apr. 16, 1940 6 Carlson July 2, 194 Luhrs July 7, 1953 Van de Lindt July 14, 1953 Heath Aug. 25, 1953 Luhrs Sept. 27, 1955 Miller Aug. 21, 1956 Harris Mar. 5, 1957 Fox Aug. 15, 1957 FOREIGN PATENTS Germany June 11, 1951 

